In ink-jet printing methods, droplets of ink are directly projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium, to form characters and images. The ink-jet printing methods have been rapidly spread because of their various advantages such as easiness of full coloration, low costs, capability of using a plain paper as the recording medium, non-contact with printed images and characters, etc. Among such printing methods, in view of enhancing a light resistance and a water resistance of the images and characters printed, an ink-jet printing method utilizing an ink containing a pigment has now come to dominate. In recent years, there is a further demand for pigment-containing inks having the same gloss as that of dye-containing inks.
There is generally known such a method for producing pigment-containing inks in which the pigment is dispersed in a liquid medium such as water and an organic solvent in the presence of a dispersant.
For example, Patent Document 1 discloses a method of dispersing a pigment together with a dispersant by means of a high-speed mill using a fine grinding medium. In the Patent Document 1, as the method of finely grinding the pigment, there are described a continuous media recirculation pulverization method and a mixed media pulverization method. In these methods, a slurry containing the fine grinding medium, liquid, pigment and dispersant is circulated through the system, and after completion of the pulverization, the resultant pigment dispersion is separated from the fine grinding medium by mechanical methods such as screening and filtration. However, in these conventional pulverization methods, the separation procedure requires a prolonged period of time, and the yield of the pigment dispersion tends to be lowered.
Also, Patent Document 2 discloses such a method of grinding a pigment together with a dispersion of a resin having a weight-average molecular weight of from 9,000 to 80,000 and further finely dividing the thus ground pigment using a homogenizer. However, in this method, since the resin used therein is a styrene/maleic acid copolymer or a styrene/acrylic acid copolymer having a weight-average molecular weight of 80,000 or less, there tends to arise such a problem that the obtained dispersion fails to exhibit excellent storage stability and gloss.    Patent Document 1: JP 9-176543A    Patent Document 2: JP 2005-41992A